You certain it’s not a setting on your TV? I just checked several stations showing old, pre-widescreen content, and I get the same aspect ratios, or nearly so, from both the TV, directly, and via the MiBox 3 using Channels.

Long running raging forest fire on Plex Forums about aspect ratios and Shield's inability to get them right:https://forums.plex.tv/search?q=aspect%20ratio%20shield
^I hope that works - a search at Plex with:
aspect ratio shield
garnered the above, but some sections of the community have circled the wagons... some time ago.

I’ve found that 4:3 aspect ratio displays correctly in Channels when using software rendering, and it correctly switches to 16:9 at ad breaks (a common occurrence in the UK). 4:3 always gets stretched when using hardware rendering.

On Nvidia Shield having it set to "Hybrid" instead of "Hardware" decoding in "Settings|Player" menu fixed the problem. Not sure what the difference is but it seemed like it would be less CPU intensive than "Software" which also worked as well.

Any updates on getting aspect ratio to switch/display correctly on the Shield with the hardware decoder? The software decoder works, but I still get random micro-judder/stutter that can only be corrected by hitting pause for a few seconds. I don't see this when using the hardware decoder.